Notice: this information was written when Microthemer version 2 was current. On Dec 26th, 2014 version 3 was released with a completely new interface. Some of the information may be redundant, but as much of
the functionality remains the same in version 3 we haven't deleted this post.
When upgrading to Microthemer 2.3.2 or 2.3.5 WordPress may deactivate the plugin and return an error message on the Plugins page saying Microthemer was deactivated because the plugin file doesn’t exist.
The solution is simple – ignore the message and reactivate Microthemer.
This happened because under some circumstances the plugin extracted to a folder called “crothemer” instead of “microthemer” (and so WordPress had the wrong file path saved in the database). Version 2.3.5 fixes this issue, but if you’re upgrading from 2.3.2 to 2.3.5 you’re going to get the same problem (as the folder has changed again from “crothemer” back to the normal “microthemer”). Just ignore the message and reactivate Microthemer. This won’t happen with future updates.